1. Field of the Invention
The present invention relates to a time-division multiplex transmission system, and in particular, to a frame synchronizing system in a receiver in the system.
2. Description of the Prior Art
In order to transmit information to different receivers from a single transmitter through a common transmission line, a time-division multiplex transmission system is used.
In the time-division multiplex transmission system, a repetition period or a time frame is divided into a plurality of time slots which are used as channels for transmitting information to different receivers, respectively.
In a certain time-division multiplex signal, a digital signal in a time slot of a channel comprises a start signal part, an address signal part, an information signal part, and a blank interval which are arrayed in this order. The start signal part comprises a pulse signal corresponding to a start bit. The address signal part comprises a digital signal representing an address of a receiver corresponding to the channel. The information signal part comprises a digital data signal representing the information to be transmitted to a corresponding receiver. The blank interval is a space between the information signal part and the subsequent channel signal.
Each receiver in the time-division multiplex transmission system has a circuit for detecting the address signal corresponding thereto from the multiplex signal so as to detect the channel signal assigned thereto.
A known address detecting circuit comprises a storage device storing an address assigned to the receiver, and a comparing circuit comparing the address in the storage device with the signal received at the receiver. When the address is detected in the received signal by the comparison, the comparing circuit generates a detection signal. Then, the receiver can discriminate its channel signal from the multiplex signal.
The address detecting circuit starts its operation by reception of the start signal part and maintaining the operation during a time period equal to the total amount of time interval of the start signal part and another time interval of the address signal part. Once the address assigned to the receiver is detected in the received multiplex signal, the operation of the address detecting circuit is thereafter repeated at a constant interval equal to the time frame.
At a start condition of the system, or at a start condition of a receiver in the system, the initial pulse applied to the address detecting circuit drives the circuit as if it is a pulse of the start signal part, so that the operation of the address detecting circuit of the receiver starts at an indefinite time position of the time frame of the multplex signal. There is a probability that an information signal part of each channel signal in the multiplex signal has a portion having the same signal pattern as the address signal part of the channel corresponding to the receiver. If the signal portion is applied to the address detecting circuit at the start condition, the address detecting circuit of the receiver erroneously detects the signal portion as the address signal part, so that the receiver cannot discriminate its channel signal from the multiplex signal. Namely, frame synchronization cannot be established in the receiver.
The present inventor already proposed a resolution of the problem in a copending patent application Ser. No. 715,509 filed on Mar. 25, 1985, which was assigned to the same assignee.
A channel detecting circuit proposed in the copending application is provided with a blank interval detecting circuit in addition to an address detecting circuit. Application of the multiplex signal to the address detecting circuit is prevented until the blank interval detection circuit initially detects one blank interval of the multiplex signal. At a start condition of the receiver, application of the multiplex signal to the address detecting circuit starts at not at an intermediate time position in one channel signal but the start signal part of a subsequent channel signal.
However, the channel detecting circuit proposed by the copending application cannot avoid an erroneous frame synchronization due to noise in the multiplex signal as transmitted.